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Could a planet support both large mammalian megafauna and non-avian dinosaurs at the same time? My worry is that bigger mammals have progressively more trouble with dissipating body heat, whereas large dinosaurs were generally cold-blooded, meaning they didn't have as much trouble. (I'm no expert so if megafauna got along fine in very hot temperatures, or vice versa for dinosaurs please feel free to correct me.)

Thinking of making a world with slightly less of a tilt than Earth's, about .7g and an atmospheric composition similar to Jurassic times (avg. Temp=300K).

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    $\begingroup$ It's thought that most dinosaurs were endotherms these days. $\endgroup$ – SealBoi Jul 5 '18 at 13:50
  • $\begingroup$ I'm not an expert either, but I'd say that mammalians need body heat and what they struggle for is to keep heat rather than dissipating it. Hence larger size is an advantage to them, among many other disadvantages $\endgroup$ – Rafael Jul 5 '18 at 13:51
  • $\begingroup$ If you specifically need land mammalian megafauna, and dinosaurs that are not birds, please include it in your question - because now our current Earth can fit your description well, if we will use favorable definitions. $\endgroup$ – Mołot Jul 6 '18 at 14:10
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Dinos are not cooler than megafauna

Well, to talk about megafauna handling the heat well... please consider the closest thing we have today, the African elephant. The daytime temperature in Kenya, one of the places where elephants live today, can easily reach around 28 C, or 301 K. So saying "megafauna can't handle the heat" looks a little baseless from where I'm standing right now.

As for dinosaurs being hot-blooded or cold-blooded, I found a paperabout this topic. The researchers, using dino teeth, were able to determine body temperature. What they found was that the temperature of larger dinos was already below previous estimates.

Here is the paper.

I'm going to quote from the abstract here:

This temperature range is 4° to 7°C lower than predicted by a model that showed scaling of dinosaur body temperature with mass, which could indicate that sauropods had mechanisms to prevent excessively high body temperatures being reached because of their gigantic size.

Large dinosaurs probably had thermal regulation, and this was irrelevant to whether they were hot- or cold-blooded. The paper authors did not take a stand on the hot-blooded versus cold-blooded debate in the paper.

An unresolved question is whether such adaptations could have compensated for the high internal heat production associated with endothermy, or whether large adult sauropods must have had both heat-dissipating adaptations and a low basal metabolism to maintain body temperatures in the 36° to 38°C range that we have measured.

I looked at a few papers and they all brought up "thermal inertia" or "inertial homeothermy." I even found a paper about such (link) in which the author argues that thermal inertia alone would have been enough for large dinosaurs.

Unless you have cooling elements, like an elephant's ears, you aren't cooling down quickly. Stegosaurus was suggested to have used its plates in a similar fashion (link.)

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    $\begingroup$ This does not have the citations needed from a 'hard-science' answer. Please see that tag below the original question. $\endgroup$ – kingledion Jul 6 '18 at 12:34
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    $\begingroup$ As @kingledion commented, Please include links to the articles and presentation you've found on the topic (and please ensure that they are corroborated by scientific publications - I've found a presentation showing that the Earth is really flat, that most governments have disguised reptilians members, and that a machine generating free energy from magnets can be built in your typical garage ;-) ). Great answer aside from missing citations btw. $\endgroup$ – G0BLiN Jul 8 '18 at 15:04
  • $\begingroup$ I've added some sources - the correct sources, mind you, since the presentation I found early was ripping off a paper by other people. Are there still problems? $\endgroup$ – ltmauve Jul 9 '18 at 17:38
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The answer is "Yes and no."

There's no a priori reason that large mammals and dinosaurs couldn't coexist. It's not obvious why one would automatically out-compete the other, especially since some dinosaurs were probably warm-blooded.

But there are some limitations, the biggest being that carnivorous beasts can never be very common and compete with each other for food. See Why Big Fierce Animals are Rare: an Ecologist's Perspective by Paul Colinvaux.

So, yes, they could co-exist, but no, you can't have all the predatory dinosaurs and all the big predatory mammals at once. You might well be able to sustain populations of many grazing animals of each type.

(And then we'd get to see in a fair match -- no cheating with space rocks! -- who out competes who.)

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Mammals have the smallest and most flexible blood cells of all vertebrates, which are made inside the mammals bones. It allows for miniature capillaries, and perhaps different sensory abilities.

Dinosaur blood cells could divide, they kept their nucleus, and were larger, and so they could have different bone structures, perhaps stronger and lighter.

Mammal bone structure may be limited by the marrow, which doesn't exist in birds and dinosaurs.

Most current research suggests that Dinosaurs, the most recent family of giant lizards, were not cold blooded. Scientists use terms like Archosaurs, to describe all large lizards. The older lizards had temperature regulation like crocodiles, and the later ones had feathers and warm blood. They can measure isotopes from teeth and bones to determine ambient temperature at time of growth, which were about 35.7>t'>38.2 in dinosaurs, warmer than crocodiles (30) and colder than birds (38) https://blog.everythingdinosaur.co.uk/blog/_archives/2011/06/20/4845668.html

You are focusing on only one factor, to say why they couldn't co-exist. Mammals were in a nocturnal ecosystem niche and only small advantages in one group of animals provides enough pressure to prevent the other group from becoming the same size and competing with them.

Other advantages can be: Faster growth rate, bipedalism(dino's had either 2 fast legs or armor/size), full body armor protection(elephants are jelly for velociraptors), live-bearing versus egg laying(pregnant and young mammals), neurological(some dino's had secondary brains in the spine), skeletal(bone marrow vs hollow bones), muscular and other factors that tailored dinosaurs to speed strength and efficiency at large sizes. Dinosaurs did live in cold forests, and the best boreal megafauna are like balls of blubber with little legs compared to dinosaurs, feathers are better protection than fat.

The reason why birds did could not return into the ocean and become masters of the seas like whales, is because they would have to lay eggs in the sea which even penguins and marine birds haven't evolved.

There were warm blooded sea-swimming dinosaurs (mososaurs) that looked like diplodocus which gave birth to live young at sea, so birds like penguins and gannets are fairly close to becoming whales again given time.

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hey blue whales are technically Megafauna , and birds are just avian dinosaurs(flying dinosaurs) and the coexist, therefore you senario could is completely plausible even if the tilt of earth was a bit different that today's.

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    $\begingroup$ This is not a hard science answer. $\endgroup$ – kingledion Jul 6 '18 at 12:34
  • $\begingroup$ Welcome to WorldBuilding Persivefire! Please have a look at the tag wiki for hard-science. Sometimes questions require very scientific answers that include citations, equations, references to formal academic papers and similar sources to be valid answers. This is such a case. There is also a yellow box under the question which states the most important criteria. Your answer currently does not look like it fulfills these requirements and as such might get deleted. Please edit your answer and explain, with sufficient scientific sources, why your answer is correct. Have fun! $\endgroup$ – Secespitus Jul 6 '18 at 12:51
  • $\begingroup$ this answer is not really supposed to be an complete answer but more like a side note or an FYI thing and most of what I wrote here is just a combination of the things that I have learned over time and research long time back AKA a brief compilation of my existing knowledge on this subject in my own words . This does does not dwell into the details as I wanted to keep it brief . $\endgroup$ – Persivefire Jul 12 '18 at 18:42

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